In the electrolysis or electrosynthesis of chemical compounds, a porous diaphragm is often used to separate the anode and cathode compartments and the reaction products, while permitting the flow of some liquid components from one compartment to another. For example, in the production of chlorine and sodium hydroxide from brine, the brine feed flows from the anode compartment through the porous diaphragm to the cathode compartment and then is discharged from the cell.
Porous diaphragms are widely used for the electrolysis of aqueous sodium chloride to make sodium hydroxide and chlorine. While asbestos has been widely used as a porous diaphragm in electrolytic cells, it is known to present serious environmental hazards and has a short lifetime.
Synthetic diaphragms are preferred to asbestos diaphragms in many respects because they can be operated at high current density and low cell voltage without being destroyed by power upsets, fluctuations or outages. Synthetic diaphragms do not present serious environmental hazards and exhibit a longer lifetime than comparable asbestos diaphragms. The synthetic diaphragm must, however, be resistant to the chemicals employed in separation devices or electrolytic cells at temperatures which frequently approach 100.degree. C. The effect of diaphragm structure on the performance of an electrolytic cell is quite complex. The diaphragm can be described in terms of pore size distribution, porosity, tortuosity, thickness and resultant permeability of the structure. For a given set of cell operating conditions, these parameters, and especially their uniformity across the active area of the diaphragm, determine the electrical energy usage of the cell. The art in designing a diaphragm for use in electrolytic cell is to properly balance the diaphragm properties to minimize overall electrical energy usage by reducing operating voltage while maintaining high current efficiency. This is most effectively done with a diaphragm whose properties are highly uniform across its active area.
Furthermore, the synthetic diaphragms, usually made of polytetrafluoroethylene (PTFE), are difficult to fully wet with the electrolyte. If the diaphragm is not fully wetted, gas bubbles generated at the electrodes will accumulate in the diaphragm pores, blocking both bulk and ion flow. This reduces the effective diaphragm area, leading to an increase in voltage and eventually causing premature shutdown of the electrolytic cell. This is called "gas locking."
U.S. Pat. No. 3,853,720 teaches the treatment of diaphragms containing both asbestos and synthetic fibers with ion exchange resins to help solve this problem.
Several patents describe porous PTFE diaphragms made by combining PTFE powder or fiber with a sacrificial filler. The mixture is formed into a sheet and the filler is removed by dissolving or decomposing it, leaving porous PTFE. These diaphragms generally operate at a high cell voltage.
Other patents describe synthetic diaphragms prepared by a PTFE fiber slurry deposition process. The size, shape and size distribution of the PTFE fiber available leads to a large pore, inherently weak, nonuniform structure which must be made very thick to provide utility. The result is high cell voltage.
Other patents describe a single-layer diaphragm made of expanded polytetrafluoroethylene (EPTFE) which is wettable by various methods, including impregnating it with a solution of a perfluoro cation exchange resin. Without wettability achieved with a chemically resistant coating, an adequate level of hydrophilicity is not maintained and "gas locking" occurs reducing the effective diaphragm area leading to an increase in operating voltage.
U.S. Pat. No. 3,944,477 describes a diaphragm of porous PTFE sheet material with a microstructure characterized by nodes and fibrils and having a multilayer structure wherein a number of such sheets are bonded together. Initial wettability is achieved by treatment with acetone and water, but treatment of the diaphragm with acetone and water alone does not provide long-term wetting. In the harsh environment of an electrolytic cell such as a chloralkali cell, the acetone and water do not provide long-term hydrophilicity and the diaphragm prematurely fails.
Other improvements in this technology which try to solve the problems of the prior art diaphragms, such as short life or gas blocking (as a result of incomplete wetting) or side-to-side non-uniformity of the diaphragm structure involve using multiple layers of EPTFE which are at least partially coated with a perfluoro ion exchange polymer. This provides more uniform flow rates across the diaphragm, resulting in improved current efficiency. The diaphragm may initially contain water soluble surfactant within its pores to enhance initial wetting. While this improvement represents an advance over the prior art, it has not been possible to reproduce satisfactory wetting without leaving microscopic air bubbles in the wetted diaphragm.
The present invention solves the problem of reproducible initial wetting of the dried diaphragm without losing any of the other desirable features of the prior art.